Master Equation Studies of the Unimolecular Decay of Thermalized Methacrolein Oxide: The Impact of Atmospheric Conditions.

Master equation simulations of the unimolecular reaction dynamics of the Criegee intermediate methacrolein oxide (MACR oxide) have been performed under a variety of temperature and pressure conditions. These simulations provide insight into how the unimolecular kinetics vary across temperatures spanning the range 288-320 K. This work has incorporated a new potential energy surface and includes the anti-to-syn and cis-to-trans conformational dynamics of MACR oxide, as well as the unimolecular reactions to form dioxirane and dioxole species. The competition between the unimolecular reactivity of MACR oxide and previously documented bimolecular reactivity of MACR oxide with water vapor is explored, focusing on how this competition is affected by changes in atmospheric conditions. The impact on the role of MACR oxide as an atmospheric oxidant of SO2 is noted.

Experimental characterization of the weakly anisotropic CN X 2Σ+ + Ne potential from IR-UV double resonance studies of the CN-Ne complex.

IR-UV double resonance spectroscopy has been used to characterize hindered internal rotor states (n(K) = 0(0), 1(1), and 1(0)) of the CN-Ne complex in its ground electronic state with various degrees of CN stretch (ν(CN)) excitation. Rotationally resolved infrared overtone spectra of the CN-Ne complex exhibit perturbations arising from Coriolis coupling between the closely spaced hindered rotor states (1(1) and 1(0)) with two quanta of CN stretch (ν(CN) = 2). A deperturbation analysis is used to obtain accurate rotational constants and associated average CN center-of-mass to Ne separation distances as well as the coupling strength. The energetic ordering and spacings of the hindered internal rotor states provide a direct reflection of the weakly anisotropic intermolecular potential between CN X (2)Σ(+) and Ne, with only an 8 cm(-1) barrier to CN internal rotation, from which radially averaged anisotropy parameters (V(10) and V(20)) are extracted that are consistent for ν(CN) = 0-3. Complementary ab initio calculation of the CN X (2)Σ(+) + Ne potential using MRCI+Q extrapolated to the complete one-electron basis set limit is compared with the experimentally derived anisotropy by optimizing the radial potential at each angle. Experiment and theory are in excellent accord, both indicating a bent minimum energy configuration and nearly free rotor behavior. Analogous experimental and theoretical studies of the CN-Ne complex upon electronic excitation to the CN B (2)Σ(+) state indicate a slightly more anisotropic potential with a linear CN-Ne minimum energy configuration. The results from these IR-UV double resonance studies are compared with prior electronic spectroscopy and theoretical studies of the CN-Ne system.

Reactive quenching of OD A 2Σ+ by H2: translational energy distributions for H- and D-atom product channels.

The H- and D-atom products from collisional quenching of OD A (2)Σ(+) by H(2) are characterized through Doppler spectroscopy using two-photon (2 (2)S ←← 1 (2)S) laser-induced fluorescence. Partial deuteration enables separation of the channel forming H + HOD products, which accounts for 75% of reactive quenching events, from the D + H(2)O product channel. The Doppler profiles, along with those reported previously for other isotopic variants, are transformed into product translational energy distributions using a robust fitting procedure based on discrete velocity basis functions. The product translational energy distribution for the H-atom channel is strongly peaked at low energy (below 0.5 eV) with a long tail extending to the energetic limit. By contrast, the D-atom channel exhibits a small peak at low translational energy with a distinctive secondary peak at higher translational energy (approximately 1.8 eV) before falling off to higher energy. In both cases, most of the available energy flows into internal excitation of the water products. Similar distributions are obtained upon reanalysis of D- and H-atom Doppler profiles, respectively, from reactive quenching of OH A (2)Σ(+) by D(2). The sum of the translational energy distributions for H- and D-atom channels is remarkably similar to that obtained for OH A (2)Σ(+) + H(2), where the two channels cannot be distinguished from one another. The product translational energy distributions from reactive quenching are compared with those obtained from a previous experiment performed at higher collision energy, quasiclassical trajectory calculations of the post-quenching dynamics, and a statistical model.

Theoretical and experimental studies of collision-induced electronic energy transfer from v=0-3 of the E(0g +) ion-pair state of Br2: collisions with He and Ar.

Collisions of Br(2), prepared in the E(0(g)+) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D', and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions.

Rovibrational resonance effects in collision-induced electronic energy transfer: I2(E,v=0-2)+CF4.

Collisions of I2 in the E(0(g)+) electronic state with CF4 molecules induce electronic energy transfer to the nearby D, beta, and D' ion-pair states. Simulations of dispersed fluorescence spectra reveal collision-induced electronic energy transfer rate constants and final vibrational state distributions within each final electronic state. In comparison with earlier reports on I2(upsilon(E)=0-2) collisions with He or Ar atoms, we find markedly different dynamics when I2, excited to the same rovibronic states, collides with CF4. Final vibrational state distributions agree with the associated Franck-Condon factors with the initially prepared state to a greater degree than those found with He or Ar collision partners and suggest that internal degrees of freedom in the CF4 molecule represent a substantial means for accepting the accompanying loss of I2 vibronic energy. Comparison of the E-->D transfer of I2 excited to the J=23 and J=55 levels of the upsilon(E)=0 state reveals the onset of specific, nonstatistical dynamics as the available energy is increased above the threshold for excitation of the low frequency nu2 bending mode of CF4.

Second OH overtone excitation and statistical dissociation dynamics of peroxynitrous acid.

The second OH overtone transition of the trans-perp conformer of peroxynitrous acid (tp-HOONO) is identified using infrared action spectroscopy. HOONO is produced by the recombination of photolytically generated OH and NO(2) radicals, and then cooled in a pulsed supersonic expansion. The second overtone transition is assigned to tp-HOONO based on its vibrational frequency (10 195.3 cm(-1)) and rotational band contour, which are in accord with theoretical predictions and previous observations of the first overtone transition. The transition dipole moment associated with the overtone transition is rotated considerably from the OH bond axis, as evident from its hybrid band composition, indicating substantial charge redistribution upon OH stretch excitation. The overtone band exhibits homogeneous line broadening that is attributed to intramolecular vibrational redistribution, arising from the coupling of the initially excited OH stretch to other modes that ultimately lead to dissociation. The quantum state distributions of the OH X (2)Pi (nu=0) products following first and second OH overtone excitation of tp-HOONO are found to be statistical by comparison with three commonly used statistical models. The product state distributions are principally determined by the tp-HOONO binding energy of 16.2(1) kcal mol(-1). Only a small fraction of the OH products are produced in nu=1 following the second overtone excitation, consistent with statistical predictions.

Franck-Condon effects in collision-induced electronic energy transfer: I2(E; v = 1,2) + He, Ar.

Collisions of I2 in the E electronic state with rare gas atoms result in electronic energy transfer to the D, beta, and D' ion-pair electronic states. Rate constants for each of these channels have been measured when I2 is initially prepared in the J = 55, nu = 1 and 2 levels in the E state. The rate constants and effective hard sphere collision cross sections confirm the trends observed when nu = 0 in the E state is initially prepared: He collisions favor population of the D state, while Ar collisions favor population of the beta state. Final state vibrational level distributions are determined by spectral simulation and are found to be qualitatively consistent with the trends in the Franck-Condon factors. The experimental distributions are also compared to the recent quantum scattering calculations of Tscherbul and Buchachenko.